JSONInit.swift

public typealias JSON = [String:Any]

public enum JSONInitializableError: Error {
    case initializationError(key: String)
}

public protocol JSONInitializable {
    associatedtype Key: RawRepresentable
    init(with json: JSON) throws
}

public extension JSONInitializable where Key.RawValue == String {
    static func value<T>(for jsonKey: Key, in jsonObject: JSON) throws -> T {
        guard let value = jsonObject[jsonKey.rawValue] as? T else {
            throw JSONInitializableError.initializationError(key: jsonKey.rawValue)
        }
        return value
    }

    static func optionalValue<T>(for jsonKey: Key, in jsonObject: JSON) -> T? {
        return try? value(for: jsonKey, in: jsonObject)
    }
}

struct Address {
    let streetName: String
    let streetNumber: Int
    let town: String
}

extension Address: JSONInitializable {
    enum Key: String {
        case streetName, streetNumber, town
    }

    init(with json: JSON) throws {
        streetNumber = try Address.value(for: .streetNumber, in: json)
        streetName = try Address.value(for: .streetName, in: json)
        town = try Address.value(for: .town, in: json)
    }
}

public struct Person {
    let name: String
    var age: Int
    var address: Address?
}

extension Person: JSONInitializable {
    public enum Key: String {
        case name, age, address
    }

    public init(with json: JSON) throws {
        name = try Person.value(for: .name, in: json)
        age = Person.optionalValue(for: .age, in: json) ?? 0
        address = try Person.optionalValue(for: .address, in: json)
            .flatMap(Address.init(with:))
    }
}